Means for limiting an output signal of an amplifier stage

ABSTRACT

An electronic circuit is provided which can autonomously handle an input current (I i ) having a relatively wide dynamic range without being overdriven. The electronic circuit comprises an amplifier stage (AMPST) having an input (IP) for receiving the input current (I i ) and an output (OP) for supplying an output current (I o ), such that, during operation, the strength of the output current (I o ) increases in response to an increasing strength of the input current (I i ) as long as the strength of the input current (I i ) has not exceeded an input reference level. The strength of the output current (I o ) is kept approximately constant when the strength of the input current (I i ) has exceeded the input reference level but has not exceeded a further input reference level. The strength of the output current (I o ) decreases in response to an increasing strength of the input current (I i ) when the strength of the input current (I i ) has exceeded the further input reference level. The amplifier stage (AMPST) may comprise a current mirror (CM) having an input which forms the input (IP), an output which forms the output (OP), and a common node (cn). The amplifier stage (AMPST) further comprises first control means (FCM) having an input connected to the input (EP), and an output connected to the common node (cn). First control means (FCM) controls a current (I 2 ) to the common node (cn) and a voltage (V cn ) at the common node (cn). The first control means (FCM) comprises limiting means (LMT) for limiting the current (I 2 ) when the value of the input current (I i ) has exceeded the input reference level. Then both the input and the output currents (I i  and I o ) are limited. In order to avoid a saturation situation of a current source (I s ) which supplies a current (I) to the input (EP), the amplifier stage (AMPST) may comprise second control means (SCM) for supplying a compensation current (I CMP ) to the input (IP) when the input signal (I i ) has exceeded the input reference level. The current mirror (CM) comprises first (CP 1 ) and second (CP 2 ) current paths which form the core of the current mirror (CM), as is generally known. The decrease in response to an increasing strength of the input current (I i ) when the strength of the input current (I i ) has exceeded the further input reference level is implemented by a third current path (CP 3 ) which takes away current from the second current path (CP 2 ). Optionally, to avoid that the value of the output current (I o ) can become too low, a fourth current path (CP 4 ) may be implemented which applies current to the second current path (CP 2 ). The inventive electronic circuit may be advantageously applied in all electronic systems (like CD-apparatus) which need means to limit a maximum output signal.

The invention relates to an electronic circuit comprising an amplifier stage having an input for receiving an input signal and an output for supplying an output signal, wherein, during operation, the strength of the output signal increases in response to an increasing strength of the input signal as long as the strength of the input signal has not exceeded an input reference level.

Such an electronic circuit is known from the general state of the art. In numerous electronic systems amplifiers are needed to amplify a signal. Generally a relatively weak input signal is converted into a relatively strong output signal. In many occasions the amplifier comprises several amplifier stages. Sometimes the dynamic range of the input signal is very large. This can cause an overdrive of one or more of the amplifiers stages. This is not always a serious problem. In fact in many applications it is appreciated that relatively strong input signals cannot be amplified into output signals above a certain desired level. When an amplifier stage is overdriven, this causes one or more bipolar transistors (if implemented in the amplifier stage) to go into saturation, or causes one or more field effect transistors (if implemented in the amplifier stage) to go out of saturation. If the input signal decreases after such an overdrive has occurred, the amplifier should quickly recover in order to fully amplify the weaker input signal within a short amount of time. However, bipolar transistors need some time to come out of saturation, and field effect transistors need some time to get into saturation again. For this reason the amplifier needs some time to recover. There are applications which require a very short recovery time of the amplifier. A very good way to guarantee a very short recovery time is to provide the amplifier with clipping means implemented in such away that the transistors which might influence the recovery time cannot go out of their normal biasing, that is to say: bipolar transistors will never go into saturation, and field effect transistors will never go out of saturation.

U.S. Pat. No. 6,108,293 discloses an optical disk recording device for reading and recording of data through irradiation by a laser beam on an optical disk medium. The device comprises a photo detection circuit for receiving input of reflected light from an optical disk medium to detect a reproduced signal. An amplifier amplifies the reproduced signal and outputs the amplified reproduced signal for monitoring. In a recording mode, high-intensity laser pulses are focused on the optical disk medium. In between these pulses, information (like in reading mode) must also be read in order to acquire tracking information and e.g. a so-called wobble signal. The high-density laser pulses cause a relatively high amplitude of the signal from the photo detection circuit which is put into the amplifier. This may overdrive the amplifier. As a consequence the recovery time of the amplifier may be too long. A way to overcome this problem is proposed by the addition of current subtracting means which subtracts current away from the photo detection circuit under control of a so-called recording gate signal. An alternative solution is proposed whereby a gain control circuit is added in between the photo detection circuit and the amplifier. The gain of the gain control circuit is then also put under the control of a recording gate signal.

A disadvantage of both proposed solutions is that they need additional information (e.g. the recording gate signal) and additional circuitry in order to avoid an overdrive of the amplifier. In other words, the amplifier cannot autonomously perform its amplifying task without the danger of being overdriven.

Therefore it is an object of the invention to provide an electronic circuit which can autonomously handle an input signal having a relatively wide dynamic range without being overdriven.

To this end, according to the invention, the electronic circuit of the type defined in the opening paragraph is characterized in that the strength of the output signal is kept approximately constant when the strength of the input signal has exceeded the input reference level but has not exceeded a further input reference level, and that the strength of the output signal decreases in response to an increasing strength of the input signal when the strength of the input signal has exceeded the further input reference level.

An overdrive of the amplifier stage can be avoided by proper definition of the input reference level. By proper definition of the input reference level is meant that input signals having an amplitude not higher than the input reference level will be amplified by the amplifier stage in a normal mode (usually this means a more or less linear amplification), that is to say without clipping. The amplifier stage is dimensioned in a way such that the transistors which might influence the recovery time of the amplifier stage remain normally biased, that is to say they will not undesirably go into or out of saturation in this normal mode. When the amplitude of the input signal is higher than the input reference level, but still not higher than the further input reference level, the amplifier stage is in a clipping mode. The output signal is kept constant in the clipping mode. The clipping is performed in a manner such that the transistors of the amplifier stage retain their normal biasing.

In principle the inventive electronic circuit may have only one amplifier stage. Usually, however, a plurality of amplifier stages is used. If, for example, two amplifier stages are used, it may be sufficient that only the first amplifier stage is provided with the said clipping means. After all, in this situation, the amplitude of the output signal of the first amplifier stage, which forms the input signal of the second amplifier stage, is already clipped when the amplitude of the input signal has exceeded the input reference level. However, it is to be emphasized that it is not necessary that the inventive amplifier stage should always be the first stage. Several inventive amplifier stages may alternatively be applied.

When the amplitude of the input signal is higher than the further input reference level, the amplifier stage is in a fold-back mode, that is to say that the amplitude of the output signal is no longer kept constant, but decreases with a further increase in the amplitude of the input signal. This has the advantage of a reduced power consumption of the electronic circuit.

An embodiment of the invention is characterized in that the strength of the output signal cannot become lower than an output reference level when the strength of the input signal has exceeded the further input reference level. It is avoided thereby that current through transistors in the amplifier stage can become very low, e.g. zero. This prevents transistors from reacting slowly. If the output signal has reached the said output reference level, this situation will be further denoted the minimum fold-back mode. Thus the recovery time of the amplifier from the (minimum) fold-back mode into the normal mode is further decreased. Any other amplifier stages following the inventive amplifier stage will also recover more quickly.

An embodiment of the invention is characterized in that the further input reference level is approximately equal to the input reference level. This means that the clipping mode is now not present. Thus when the amplitude of the input signal exceeds the input reference level, the amplifier immediately enters the fold-back mode. This has the advantage that the average power consumption of the amplifier stage is further reduced. Another advantage is the somewhat less complex implementation of the amplifier stage.

The presence of a clipping mode, however, may be advantageous. If, for example, the dynamic range of the input signal is such that it just does not exceed the input reference level, then the presence of some noise, which is unfortunately always present in a signal, may cause the input reference signal to be exceeded during the positive values of the noise and not during the negative values of the noise. This causes a distortion of the output signal because HF-noise can be converted into LF-noise in this case. Also a DC-component is introduced by this clipping. If there is no clipping mode in such a situation, however, the amplifier will enter into the fold-back mode during the positive values of the noise. This causes the same distortion as previously described, but at a much higher level. Thus it depends on the design parameters of the electronic circuit whether or not it is desirable to implement a clipping mode in the amplifier stage.

An embodiment of the invention is characterized in that the input signal is an input current, and the output signal is an output current. In practice the invention can most easily be implemented in the current domain. Then the core of the amplifier stage can be implemented by a (simple) current mirror which may have a current mirror gain factor.

Alternatively, the invention may be implemented in the voltage domain. This is even possible when the input signal is a current, and an output signal of the electronic circuit is also a current. It is then possible to put the input signal into a current-to-voltage converter, apply the inventive principles in the voltage domain, and then convert the voltage back into a current by a voltage-to-current converter.

An embodiment of the invention in which the input and output signals of the amplifier stage are currents is characterized in that the amplifier stage comprises a first current path coupled between the input and a common node; a second current path coupled between the output and the common node; first control means coupled between the input and the common node for controlling a voltage on the common node and for supplying a current to the common node, the first control means comprising limiting means for limiting the current to the common node when the strength of the input signal has exceeded the input reference level; and second control means for supplying a compensation current to the input when the strength of the input signal has exceeded the input reference level. The first and second current paths together form a current mirror. Usually a current mirror has an input, an output, and a common reference which is usually connected to a power supply terminal. In this case the input of the current mirror thus formed is in fact the input of the amplifier stage, and thus receives current from a signal current source. The output of the current mirror is in fact the output of the amplifier stage. The common reference, which is denoted the common node, is not connected to a power supply terminal in this invention but to the first control means. This provides the possibility of not only controlling the voltage at the common node, but also of limiting the maximum current to the common node. If the input current level becomes higher than the input reference level, then the current to the common node is limited by the limiting means within the first control means. In that situation the current from the signal current source can no longer be fully put into the input of the amplifier stage. This would cause a saturation situation of the signal current source, whereby the signal current source is forced to deliver less current. Generally this is an unfavorable situation. This situation is avoided, however, by the presence of the second control means, which supplies a compensation current to the input when the strength of the input signal has exceeded the input reference level.

An embodiment of the invention is characterized in that the amplifier stage futher comprises a third current path having a first side coupled to the input and a second side coupled to the second current path for taking away current from the second current path, such that the strength of the output current decreases in response to an increasing strength of the input signal when the strength of the input signal has exceeded the further input reference level. This is an example of an implementation of the fold-back mode. The taking away of part of the current is not accomplished by a separate current means but by using (part of) the residual portion of the current delivered by the signal current source. This reduces the power consumption of the amplifier stage.

An embodiment of the invention is characterized in that the amplifier stage further comprises a fourth current path coupled to the second current path for supplying current to the second current path in order to avoid that the output current can be lower than the output reference level when the strength of the input signal has exceeded the further input reference level. This is an example of an implementation of the minimum fold-back mode. The coupling of the fourth current path to the second current path may coincide with the second side of the third current path. This is, however, not a necessity.

The invention also relates to an optical/magneto-optical disk recording apparatus. An inventive optical/magneto-optical disk recording apparatus having a light source for storing data on a disk, and light-receiving means for the detection of data from the disk, is characterized in that it comprises the inventive electronic circuit wherein the input signal of the amplifier stage is responsive to a signal delivered by the light receiving means.

The light source is usually a laser. Light pulses with a high intensity are used to write data on an optical disk. Such a disk is, for example, a CD (Compact Disk), DVD (Digital Versatile Disk), or BD (Blu-ray Disk, formerly denoted DVR). The apparatus comprises a so-called PDIC (Photo Diode Integrated Circuit) which is an IC having (pre-) amplifiers and integrated photo diodes, the latter acting as the light-receiving means for the detection of data from the disk. The PDIC is used to monitor the reading/writing process. In order not to overdrive the PDIC during the writing process, a low amplifier gain is needed. However, the signal parts in between the high intensity light pulses must also be read because they contain servo information, track addresses, and a wobble signal. These signal parts cannot be processed with a low amplifier gain, however, because these signals parts would then drown in offset and noise. This is because these signal parts have in fact a small amplitude (comparable to the amplitude of the signal during the reading process).A high amplifier gain is therefore needed.

A high amplifier gain is thus needed during the writing process for the processing of the weak signal parts in between the high intensity light pulses on the one hand, and on the other hand a low amplifier gain is needed to monitor the (reflected) high intensity light pulses without overdriving the PDIC. It is possible in theory to switch between a low and a high gain path. In practice this turns out to be difficult to implement because it imposes very high settling requirements on amplifiers stages.

The implementation of the inventive electronic circuit overcomes this difficulty because it comprises the inventive amplifier stage which can handle a very wide dynamic range of the input signal without being overdriven.

The inventive electronic circuit can be advantageously applied in all electronic systems which need means for limiting an output signal and which need a very short recovery time when the relatively strong input signal is reduced to a relatively weak input signal. The electronic circuit may thus be used, for example, in the high-frequency part of a receiver (like radio, television), where clipping of strong signals does not cause distortion of the information, for example when frequency or phase modulation is used. If the receiver is of the very commonly used “super heterodyne type”, then the inventive amplifier stage may also be implemented in the so-called intermediate-frequency part.

The invention also includes a method whereby an input signal is converted into an output signal, and whereby the strength of the output signal increases in response to an increasing strength of the input signal as long as the strength of the input signal does not exceed an input reference level, and whereby the strength of the output signal is kept approximately constant when the strength of the input signal exceeds the input reference level but does not exceed a further input reference level, and whereby the strength of the output signal decreases in response to an increasing strength of the input signal when the strength of the input signal exceeds the further input reference level.

Optionally, the further input reference level may be chosen to be equal to the input reference level. This has the effect that the output signal immediately decreases in response to an increasing strength of the input signal when the strength of the input signal has exceeded the input reference level.

An embodiment of the inventive method is characterized in that the strength of the output signal does not become lower than an output reference level when the strength of the input signal exceeds the further input reference level.

The invention will be described in more detail with reference to the accompanying drawings, in which:

FIG. 1 is a simplified diagram of an optical disk drive apparatus in which the invention can be advantageously applied;

FIG. 2 show different gain paths for the handling of a signal from photo diodes;

FIG. 3 shows a signal diagram of the laser light power and the reflected laser light power of the optical disk drive apparatus in the recording mode;

FIGS. 4 and 5 show signal diagrams for explaining the principles of the invention;

FIGS. 6 and 7 are diagrams showing parts of an inventive amplifier stage for giving an introductory explanation of the invention in broad terms;

FIG. 8 shows an embodiment of the inventive amplifier stage in broad terms;

FIGS. 9-13 are detailed electronic diagrams of parts of the inventive amplifier stage; and

FIGS. 14-16 are detailed electronic diagrams of embodiments of the inventive amplifier stage.

In these Figures, parts or elements having like functions or purposes bear the same reference symbols.

FIG. 1 is a simplified diagram of an optical disk drive recording/reading apparatus comprising a light source LS, a half-prism HP, an objective lens OL, photo diodes PHDS, and electronic processing means PR.

In a reading mode, information stored on an optical (or magneto-optical) disk DSK must be retrieved. The light source LS, which is usually a laser device, radiates a light beam which passes partly through the half-prism HP. The light is then focused on an information layer of the DSK by the objective lens OL. The information layer may comprise so-called pits and lands. These pits and lands are in fact logic “0” and “1” values representing the information. Thus the information is stored in a binary (and digital) form. Light incident on a land is strongly reflected back. Light incident on a pit is also reflected back, but to a lesser degree. Therefore the stored logic “0” and “1” values can be retrieved by detection of the reflected light from the disk DSK. The reflected light is partly mirrored and is received by light-receiving means. The light-receiving means are, for example, implemented by the photo diodes PHDS. Usually the photo diodes PHDS comprise several parts for delivering separate signals to the processing means PR. In this example the photo diodes PHDS comprise 4 parts, and thus 4 separate signals A, B, C, and D are delivered. In many cases the photo diodes PHDS and the processing means PR are constructed in a single IC (integrated circuit) which is often denoted as PDIC (Photo Diode Integrated Circuit). The processing means PR retrieves the information (usually) through summation of the 4 signals A, B, C, and D. Other information is also acquired by various combinations of the 4 signals A, B, C, and D. Thus servo information, track addresses, wobble signals, etcetera can be retrieved in this manner as well.

There are several ways to register information on the disk in the recording (writing) mode. Instead of using the method of actually creating pits and lands, which method is normally used in the manufacture of ROM-disks, other methods are normally used for recordable disks. The disk DSK is provided, for example, with a dye which partly reflects light. During recording the reflectivity of the dye is reduced as a result of the focusing of high-intensity laser pulses L₁ (see FIG. 3) on the optical disk DSK Logic “0” and “1” values can be stored and may (later) be retrieved through detection of the reflected light from the disk DSK. Instead of using a dye as the information layer of the disk DSK, alternative principles for registering information may also be applied, for example the so called “phase change principle”. In this latter principle a crystalline material is converted into an amorphous material (or vice versa) as the result of the focusing of the high-intensity laser pulses L₁ on the optical disk DSK.

The recording mode will be further explained with reference to FIGS. 1 and 3. High intensity laser pulses L₁ are focused on the optical disk DSK The maximum value of the laser power LP of the laser pulses L₁ is denoted WL (“Write Level”). The reflected light power is denoted light “pulse” L₂. During a laser pulse L₁ the reflected light “pulse” L₂ decreases. See, for example, the time period between moments t₁ and t₂. This provides the possibility of controlling the reflectivity by measuring the reflected light “pulses” L₂. In between the laser pulses L₁, information (like in reading mode) must also be read in order to acquire tracking information, etcetera. So this information is read, for example, between moments t₂ and t₃. The value of the laser power LP of the laser pulses L₁ is then much less. This value is denoted RL (“Read Level”).

In the recording mode, therefore, reflected light with high intensity as well as reflected light with low intensity must be processed by the processing means PR. As a consequence the dynamic range of one or more of the 4 signals A, B, C, and D may be very wide. This can cause problems with amplifier stages implemented in the processing means PR, because it imposes very high settling requirements on the amplifier stages. For further elucidation of this problem, which is solved by the application of the invention, reference is now also made to FIG. 2. Only the signal A delivered by the photo diodes PHDS is further discussed by way of example. The recording mode of the optical disk drive apparatus will only be discussed below.

FIG. 2 shows a first gain path GPTH₁ having a relatively high gain factor G₁, and a second gain path GPTH₂ having a relatively low gain factor G₂. The first gain path GPTH₁ serves to amplify the weak signals (at “RL” level) in between the high-intensity laser pulses L₁. The second gain path GPTH₂ serves to amplify the reflected light “pulses” L₂ which have a much greater intensity. During the processing of the strong signals, for example between moments t₁ and t₂, one or more amplifier stages in the first gain path GPTH₁ will be overdriven. Unless special measures are taken, this will cause one or more bipolar transistors (if implemented) to go into saturation, or cause one or more field effect transistors (if implemented) to go out of saturation. If the amplitude of signal A decreases after such an overdrive has occurred, for example between moments t₂ and t₃, the first gain path GPTH₁ should recover quickly in order to amplify the weak signal A fully within a short amount of time. However, bipolar transistors need some time to come out of saturation, and field effect transistors need some time to go into saturation again. For this reason the first gain path GPTH, needs some time to recover. This recovery time reduces the maximum recording speed of the optical disk drive apparatus, and is thus undesirable.

The maximum recording speed can be increased significantly by the application of the inventive electronic circuit having at least one inventive amplifier stage which is implemented in the first gain path GPTH₁.

FIG. 4 shows a signal diagram for clarifying the principles of the invention. The signal diagram represents the signal transfer function G₁ of an inventive amplifier stage in which the signal A forms the input signal of the amplifier stage and G₁·A is the output signal of the amplifier stage. As long as the amplitude of the input signal A is no higher than an input reference level I_(A), the amplifier stage is in a so-called normal mode. The output signal G₁·A is then approximately constant in this example. Thus the input signal A is linearly amplified. (It is to be noted, however, that a linear amplification in this normal mode, although desirable in many applications, is not a necessity.) The value of the input reference level I_(A) must be chosen such that it is equal to or higher than the maximum amplitude of the weak signals (at “RL” level) in between the high-intensity laser pulses L₁.

During a laser pulse L₁ the reflected light “pulse” L₂ which is now represented by the input signal A has an amplitude which is higher than the input reference level I_(A). (These reflected light “pulses” L₂ are monitored via the second gain path GPTH₂.) If the amplitude is not higher than a further input reference level I_(B), however, the amplifier stage is in a so called clipping mode. The output signal G₁·A is then approximately kept constant in this example. If the amplitude of the input signal A exceeds the further input reference level I_(B), the amplifier stage is in a so-called fold-back mode. Preferably, in the fold-back mode, a minimum value for the amplitude of the output signal G₁·A must be set. This minimum value is denoted an output reference level Io_(mn). When the amplitude of the input signal A is so strong that the amplitude of the output signal has reached the output reference level Io_(mn), the amplifier stage is in a so-called minimum fold-back mode. Thus this occurs when the input signal A exceeds an even further input reference level I_(C).

FIG. 5 shows a signal diagram which differs from the signal diagram as shown in FIG. 4 in that the further input reference level I_(B) equals the input reference level I_(A). The amplifier stage then immediately enters the (minimum) fold-back mode when the input signal A exceeds the input reference level I_(A).

FIG. 6 shows an amplifier stage AMPST comprising a current mirror CM having an input IP, an output OP, and a common node cn. The current mirror CM comprises a first current path CP₁ coupled between the input IP and the common node cn and a second current path CP₂ coupled between the output OP and the common node cn. The amplifier stage AMPST further comprises first control means FCM which comprises limiting means LMT. The first control means FCM is connected between the input IP and the common node cn. An input current source J_(S) which supplies a current I_(i) is connected to the input IP. The input current source J_(S) represents, for example, the photo diodes PHDS (see FIG. 1) such that the current I_(i) represents a combination of the 4 signals A, B, C, and D, for example the signal A. The first control means FCM controls a voltage V_(cn) at the common node cn and supplies a current I₂ to the common node cn. The current I₂ feeds the first and second current paths CP₁ and CP₂. Since, in this example, an input current I₁ of the first control means FCM equals zero, the value of the current in the first current path CP₁ equals the value of the current I_(j). The current I_(o) in the second current path CP₂ forms the output current of the amplifier stage AMPST.

If the amplifier stage AMPST is in the normal mode, the current I_(i) is (linearly) amplified into the output current I_(o). The value of the amplification is determined by a so-called current mirror ratio which is defined by the current paths CP₁ and CP₂. When the amplifier stage AMPST enters the clipping mode (or the fold-back mode in the situation as shown in FIG. 5) the limiting means LMT limits the value of the current I₂. This occurs when the amplitude of the input signal I_(i) exceeds the input reference level I_(A) (see FIGS. 4 and 5). As a consequence both the currents I_(i) and I _(o) are limited. The maximum value for the current I_(o) corresponds to the value Io_(mx) as indicated in FIGS. 4 and 5. Since the current I_(i) is also limited (and the current I₁ equals zero), the input current is also limited and is thus no longer fully dictated by the current source J_(S). It means in fact a saturation situation of the signal current source J_(S).

FIG. 7 shows an amplifier stage AMPST which differs from the one shown in FIG. 6 in that it further comprises second control means SCM for supplying a compensation current I_(cmp) to the input IP when the strength of the input signal I_(i) has exceeded the input reference level I_(A). This prevents said saturation situation of the signal current source J_(S).

FIG. 8 shows an amplifier stage AMPST which differs from the one shown in FIG. 7 in that it further comprises a third current path CP₃ having a first side coupled to the input IP and a second side coupled to the second current path CP₂; and a fourth current path CP₄ coupled to the second current path CP₂.

The third current path CP₃ takes away current from the second current path CP₂ so that the amplitude of the output current I_(o) decreases in response to an increasing strength of the input signal I_(i) when the amplifier stage AMPST is in the fold-back mode. This occurs when the amplitude of the input signal I_(i) has exceeded the further input reference level I_(B) (see FIGS. 4 and 5).

The fourth current path CP₄ supplies current to the second current path CP₂ when the amplitude of the input signal I_(i) has exceeded an even further input reference level I_(C) (see FIGS. 4 and 5). This implements the minimum fold-back mode in which the value of the output current I_(o) cannot become lower than the output reference level Io_(mn).

FIG. 9 shows an electronic circuit comprising an amplifier stage AMPST which comprises the first control means FCM and the current mirror CM having the input IP, the output OP, and the common node cn. The amplifier stage AMPST is powered by a voltage source V_(S) which is connected between a first power supply terminal V_(DD) and a second power supply terminal V_(SS) of the amplifier stage AMPST. Input and output currents of the amplifier stage AMPST are denoted I_(i) and I_(o), respectively. The current mirror CM comprises bipolar transistors T₁ and T₂, each having a base, an emitter, and a collector, resistors R₁ and R₂; and capacitors C₁ and C₂. The bases of transistors T₁ and T₂ are connected to a reference terminal B_(RF). A voltage source V₁ is connected between the reference terminal B_(RF) and the second power supply terminal V_(SS). The collectors of transistors T₁ and T₂ are connected to the input IP and output OP, respectively. The resistor R₁ is connected between the common node cn and the emitter of transistor T₁. The resistor R₂ is connected between the common node cn and the emitter of transistor T₂. The capacitor C₁ is connected between the common node cn and the collector of the transistor T₁. The capacitor C₂ is connected between the common node cn and the collector of the transistor T₂. Transistor T₁ and resistor R₁ (and optionally also capacitor C₁) together form the first current path CP_(i). Transistor T₂ and resistor R₂ (and optionally also capacitor C₂) together form the second current path CP₂.

The first control means FCM comprises bipolar transistor T₃ having a base, an emitter, and a collector; field effect transistor M₁ having a gate, a source, and a drain; resistor R₃; current source J₁; and buffer VB1 having an input and an output. The drain of transistor M₁ is connected to the first power supply terminal V_(DD). The gate of transistor M₁ is connected to the input IP. The resistor R₃ is connected between the source of transistor M₁ and the second power supply terminal V_(SS). The base of transistor T₃ is connected to the source of transistor M₁. The emitter of transistor T₃ is connected to the second power supply terminal V_(SS). The input of buffer VB1 is connected to the collector of transistor T₃. The output of buffer VB1 is connected to the common node cn. The current source J₁ is connected between the first power supply terminal V_(DD) and the input of the buffer VB1.

The voltage source V₁ serves to supply a DC biasing voltage at the bases of transistors T₁ and T₂. Transistor M₁ and resistor R₃ together form a so-called source follower. The current mirror CM, the source follower, transistor T₃ which is biased by the current source J₁, and the buffer VB1 together form a so-called feedback loop. This feedback loop controls the voltage V_(cn) at the common node cn. It also controls the current I₂ to the common node cn. The collector current of transistor T₁ equals the input current I_(i). Preferably, the current mirror CM is dimensioned as follows: the emitter area of transistor T₂ is M times the emitter area of transistor T₁, the value of resistor R₁ is M times the value of resistor R₂, and the value of capacitor C₂ is M times as the value of capacitor C₁. Capacitors C₁ and C₂ improve the HF-behaviour of the current mirror CM. If the output OP is terminated with a relatively low impedance, especially for HF, (for example because it is coupled to another current mirror), then the value of the output current I_(o), is approximately M times the value of the input current I_(i). (M may be smaller than, equal to, or greater than 1) The current I₂ is limited by the buffer VB1 in order to create the clipping mode.

FIG. 10 shows an electronic circuit comprising an amplifier stage AMPST which differs from the one shown in FIG. 9 in that it further comprises a buffer VB2 having an input connected to the collector of transistor T₃, and an output, and in that the capacitor C₁ is connected to the output of the buffer VB2 instead of to the common node en. Furthermore, an implementation of the buffer VB1 is disclosed.

The buffer VB1 comprises an OTA (operational transconductance amplifier) having a non-inverting input connected to the collector of transistor T₃, an inverting input connected to the common node cn, and an output; fourth, fifth, and sixth bipolar transistors T₄-T₆ each having a base, an emitter, and a collector; a voltage source V₂ connected between the base of transistor T₆ and the second power supply terminal V_(SS); a current source J₂ connected between the emitter of transistor T₄ and the second power supply terminal V_(SS); and a current source J₃ connected between the first power supply terminal V_(DD) and the emitter of transistor T₅. The collectors of transistors T₅ and T₆ are connected to the second power supply terminal V_(SS). The emitter of transistor T₆ and the base of transistor T₄ are connected to the output of the OTA. The base of transistor T₅ is connected to the emitter of transistor T₄. The emitter of transistor T₅ is connected to the common node cn. The collector of transistor T₄ is connected to the first power supply terminal V_(DD).

Bipolar transistors T₄ and T₅, together with current sources J₂ and J₃ which supply current through the transistors T₄ and T₅, and the OTA together form a so-called improved “emitter follower”. (The improvement relates to the operation of the “emitter follower” even at very low currents.) The current source J₃ also supplies the current I₂ to the common node cn. In the normal mode a voltage VC at the collector of transistor T₃ equals the voltage V_(cn) at the common node cn. In this example the limiting means LMT within the first control means FCM is performed by the current source J₃.

In the clipping mode the voltage at the non-inverting input, the inverting input, and the output of the OTA rises significantly. In order to prevent a saturation situation of the OTA, the maximum voltage at the output of the OTA is limited by the presence of transistor T₆ and the voltage source V₂. This is because the transistor T₆ will significantly conduct current if its base-emitter voltage is more than approximately 0.6 V. (This voltage can be slighty different depending on the type of transistor used.) So if, for example, it is undesirable that the voltage at the output of the OTA exceeds 2.5 V, the voltage which must be supplied by the voltage source V₂ must not be larger than 1.9 V (2.5 V-0.6 V).

For HF, the buffer VB1 is bypassed by the buffer VB2 and the capacitor C₁. The feedback loop is kept stable thereby, that is to say no slew rate oscillations will occur in the clipping mode, neither will they in the normal mode when the amplifier stage has come out of the clipping mode.

FIG. 11 shows an electronic circuit comprising an amplifier stage AMPST which differs from the one shown in FIG. 10 in that it further comprises second control means SCM comprising bipolar transistors T_(8a) and T_(8b), each having a base, an emitter, and a collector; a voltage source V₃; and a voltage source V₄. The emitters of transistors T_(8a) and T_(8b) are connected to the collector of transistor T₃. The collector of transistor T_(8a) is connected to the second power supply terminal V_(SS). The collector of transistor T_(8b) is connected to the input IP. The bases of transistors T_(8a) and T_(8b) are connected to each other. The voltage source V₃ is connected between the reference terminal B_(RF) and the bases of transistors T_(8a) and T_(8b). The voltage source V₄ is connected at one side to the inverting input of the OTA and the emitter of transistor T₅, and to the current source J₃ and the common node cn at another side.

The purpose of the additional components with regard to the circuit of FIG. 10 is as follows. When the amplifier stage AMPST enters the clipping mode, the so-called small signal loop gain of the feedback loop drops to a very low value. Without the presence of the second control means SCM the voltage VC would tend to rise towards the potential at the first supply terminal V_(DD). This would turn the current source J₁ into saturation. This is prevented, however, by the second control means SCM. When the voltage VC exceeds a certain value, the transistors T_(8a) and T_(8b) start to conduct current. This has the effect that the rise of the voltage VC is restrained. The collector current of transistor T_(8b) is used as the compensation current I_(cmp) (see FIGS. 7 and 8). The dimensioning of the transistors T_(8a) and T_(8b) (e.g. the emitter ratio ax/bx=a/b as indicated in FIG. 11) and the voltage delivered by the voltage source V₃ are preferably such that the input reference level I_(A) (see FIGS. 4 and 5) is still determined by the current I₂. With the presence of the voltage source V₄ the voltages V_(cn) and the voltage VC can be chosen differently. A higher degree of flexibility in dimensioning the amplifier stage AMPST is made possible thereby.

The second control means SCM in fact reduces the small signal loop gain of the feedback loop when the amplifier stage AMPST enters the clipping mode. The amount of reduction of the small signal loop gain is determined by the emitter ratio a/b. The higher the emitter ratio a/b, the higher the reduction in the small signal loop gain. A high reduction of the small signal loop gain is favorable for the stability of the feedback loop. It therefore also reduces HF-peaking in the transfer (current gain) of the amplifier stage AMPST. However, if the emitter ratio a/b is chosen too high, the current flowing through transistor T₃ will be too low, e.g. equal to zero. This would undesirably increase the recovery time of the amplifier stage AMPST.

FIG. 12 shows an electronic circuit comprising an amplifier stage AMPST which differs from the one shown in FIG. 11 in that it further comprises a bipolar transistor T₉ having a base, an emitter connected to the input IP, and a collector connected to the first power supply terminal V_(DD); and a voltage source V₅ connected between the base of the transistor T₉ and the second power supply terminal V_(SS).

Transistor T₉ functions as an input clamp for clamping an input voltage V_(i) at the input 1P. Thus the input voltage V_(i) cannot become lower than the voltage delivered by the voltage source V₅ minus the base-emitter voltage of transistor T₉. As a consequence the base-emitter voltage of transistor T₃ cannot become too low, and a too low current through transistor T₃ is thus prevented. It means in fact that the input voltage V_(i) is reduced by a certain amount when the amplifier stage AMPST is not in the normal mode.

The voltage delivered by the voltage source V₅ is preferably dimensioned such that, when the amplifier stage AMPST is in the normal mode, the transistor T₉ virtually does not conduct any current. In order to achieve this it is recommended to track the voltage delivered by the voltage source V₅ over temperature effects and/or processing effects (if the amplifier stage AMPST is implemented in an IC). The voltage source V₅ may be implemented, for example, by stacking two “diode voltages” and a gate-source voltage of a field effect transistor. The two “diode voltages” may be implemented, for example, by a so called “V_(be) multiplier”.

FIG. 13 shows an electronic circuit comprising an amplifier stage AMPST which differs from the one shown in FIG. 12 in that the voltage source V₅ is connected to the second power supply terminal V_(SS) not directly, but via a resistor R₄, and in that the collector of transistor T_(8a) is not connected to the second power supply terminal V_(SS) but to a common junction of the voltage source V₅ and the resistor R₄.

The said reduction of the input voltage V_(i) by a certain amount when the amplifier stage AMPST is not in the normal mode according to the electronic circuit as shown in FIG. 12 in fact means a voltage variation in the input voltage V_(i). In principle this can increase the recovery time of the amplifier stage AMPST. Preferably, it should therefore be avoided. This is accomplished by the electronic circuit of FIG. 13. This is because the collector current of transistor T_(8a) causes a voltage drop across the resistor R₄. As a consequence the voltage at the base of transistor T₉ is increased. Thus the required base-emitter voltage of transistor T₉ is now realized by increasing the voltage at the base of transistor T₉ instead of reducing the value of the input voltage V_(i).

Alternatively, the order of the voltage source V₅ and the resistor R₄ may be reversed. Then the collector of transistor T_(8a) should be connected to the base of the transistor T₉.

FIG. 14 shows an electronic circuit comprising an amplifier stage AMPST which differs from the one shown in FIG. 13 in that the resistor R₂ is now represented by a series arrangement of resistors R_(2a) and R_(2b), and in that the collector of transistor T₉ is now not connected to the first power supply terminal V_(DD) but to a common junction of the resistors R_(2a) and R_(2b).

The third current path CP₃, and thus the fold-back mode is thus implemented. When the amplitude of the input current I; exceeds the further input reference level I_(B) (see FIGS. 4 and 5), the transistor T₉ takes away current from the second current path CP₂. As a consequence the value of the output current I_(o) decreases with a further increasing value of the input current I_(i). The ratio of the values of the resistors R_(2a) and R_(2b) is preferably to be determined such that the transistor T₉ will never go into saturation. It may turn out that an appropriate value of the resistor R_(2b) equals zero. In that case the collector of the transistor T₉ is in fact connected to the common connection point of the resistor R₂ and the emitter of the transistor T₂ (see FIG. 13).

In order to implement the minimum fold-back mode in which the value of the output current cannot become lower than a minimum value, which minimum value is referred to as the output reference level Io_(mn) (see FIGS. 4 and 5), some additional measure has to be taken. This measure could be, for example, that the ratio of the values of the resistors R_(2a) and R_(2b) is chosen such that at a certain value of the output current I₀ (which is the output reference level Io_(mn)) the collector-emitter voltage of the transistor T₉ has become so low that the transistor T₉ goes into saturation, and thus its collector current cannot increase any further. In practice, however, it is not always possible to find an appropriate ratio of the values of the resistors R_(2a) and R_(2b) for getting this result without conflicting with other dimensioning requirements for the amplifier stage AMPST. Furthermore, as was noted above, a saturation situation of the transistor T₉ is not a desirable situation since it may increase the recovery time of the amplifier stage AMPST.

In view of the above, another measure for the implementation of the minimum fold-back mode is disclosed in the electronic circuit of FIG. 15.

FIG. 15 shows an electronic circuit comprising an amplifier stage AMPST which differs from the one shown in FIG. 14 in that it further comprises a transistor T₁₀ having a base, an emitter coupled to the second current path CP₂ (in this example the emitter is connected to the collector of the transistor T₉), and a collector connected to the first power supply terminal V_(DD); a buffer VB3 having an input connected to the reference terminal B_(RF), and an output; and a voltage source V₆ connected between the base of the transistor T₁₀ and the output of the buffer VB3.

The fourth current path CP₄, and thus the minimum fold-back mode, is implemented thereby. The voltage delivered by the voltage source V₆ is chosen to be such that, when the amplitude of the output current I₀ tends to become lower than the output reference level Io_(mn), the transistor T₁₀ delivers a current (whose value increases with a further increase in value of the input current I_(i)) to the second current path CP₂. As a consequence the value of the output current I_(o) remains approximately constant.

The manner in which the minimum fold-back mode is implemented in the circuit according to FIG. 14 at the same time also determines a minimum value Io_(mn) of the output current I_(o) in the normal mode. For some applications this is not a problem.

FIG. 16 shows an electronic circuit comprising an amplifier stage AMPST which differs from the one shown in FIG. 15 in that it further comprises a resistor R₅ connected between the output of the buffer VB3 and the voltage source V₆; and in that the second control means SCM further comprises a bipolar transistor T_(8c) having a base connected to the base of the transistor T_(8a), an emitter connected to the emitter of the transistor T_(8a), and a collector connected to a common connection point of the voltage source V₆ and the resistor R₅. Optionally (not shown in FIG. 16), an additional voltage source may be arranged in series with the buffer VB3.

The transistor T_(8c) delivers a current when the amplifier stage is not in the normal mode. A voltage across the resistor R₅ is created thereby. This increases the voltage at the base of transistor T₁₀. As a consequence the transistor T₁₀ conducts current during the minimum fold-back mode. When, however, the amplifier stage is in the normal mode, the transistor T_(8c) does not deliver any current. As a consequence there is virtually no voltage across the resistor R₅, and thus the transistor T₁₀ does not conduct any current (provided the value of the voltage delivered by the voltage source V₆ is not chosen too large).

Thus the fourth current path CP₄ is inactive in the normal mode, and the value of the output current is not restricted to the minimum value Io_(mn) in the normal mode.

It is to be emphasized that many variations may be applied to the implementations of the disclosed electronic circuits. For instance, P-type transistors may be replaced by N-type transistors and vice versa It may then be necessary to adapt the polarities of the voltages and currents delivered by the voltage sources and the current sources, respectively. Furthermore, other types of transistors may be suitable. Basically all the indicated types of transistors maybe replaced by other types of transistors. For instance, transistors T₁ and T₂ may be replaced (both) by field effect transistors. The transistor M₁ may be replaced by a bipolar transistor. The current mirror CM may also be implemented by more sophisticated current mirrors, for instance current mirrors which additionally use cascode transistors (for cascoding the transistors T₁ and T₂), or which are provided with feedback means for the purpose of reducing the input impedance and/or increasing the output impedance.

Furthermore, the electronic circuits may be implemented in an IC or may be fully/partly constructed with discrete components.

The electronic circuits may be applied in all kinds of apparatus and systems which need means to limit an output signal and which need a very short recovery time when the relatively strong input signal is reduced to a relatively weak input signal. 

1. An electronic circuit comprising an amplifier stage (AMPST) having an input (IP) for receiving an input signal (I_(i)) and an output (OP) for supplying an output signal (I_(o)), wherein, during operation, the strength of the output signal (I_(o)) increases in response to an increasing strength of the input signal (I_(i)) as long as the strength of the input signal (I_(i)) has not exceeded an input reference level (I_(A)), characterized in that the strength of the output signal (I_(o)) is kept approximately constant when the strength of the input signal (I_(i)) has exceeded the input reference level (I_(A)) but has not exceeded a further input reference level (I_(B)), and that the strength of the output signal (I_(o)) decreases in response to an increasing strength of the input signal (I_(i)) when the strength of the input signal (I_(i)) has exceeded the further input reference level (I_(B)).
 2. An electronic circuit according to claim 1, characterized in that the strength of the output signal (I_(o)) cannot become lower than an output reference level (Io_(mn)) when the strength of the input signal (I_(i)) has exceeded the further input reference level (I_(B)).
 3. An electronic circuit according to claim 1 characterized in that the further input reference level (I_(B)) is approximately equal to the input reference level (I_(A)).
 4. An electronic circuit according to claim 1, characterized in that the input signal (I_(i)) is an input current (I_(i)), and the output signal (I_(o)) is an output current (I_(o)).
 5. An electronic circuit according to claim 4, characterized in that the amplifier stage (AMPST) comprises a first current path (CP₁) coupled between the input (IP) and a common node (cn); a second current path (CP₂) coupled between the output (OP) and the common node (cn); first control means (FCM) coupled between the input (IP) and the common node (cn) for controlling a voltage (V_(cn)) at the common node (cn) and for supplying a current (I₂) to the common node (cn), the first control means (FCM) comprising limiting means (LMT) for limiting the current (I₂) to the common node (cn) when the strength of the input signal (I_(i)) has exceeded the input reference level (I_(A)); and second control means (SCM) for supplying a compensation current (I_(cmp)) to the input (IP) when the strength of the input signal (I_(i)) has exceeded the input reference level (I_(A)).
 6. An electronic circuit according to claim 5, characterized in that the amplifier stage (AMPST) further comprises a third current path (CP₃) having a first side coupled to the input (IP) and a second side coupled to the second current path (CP₂) for taking away current from the second current path (CP₂), such that the strength of the output current (I_(o)) decreases in response to an increasing strength of the input signal (I_(i)) when the strength of the input signal (I_(i)) has exceeded the further input reference level (I_(B)).
 7. An electronic circuit according to claim 6, characterized in that the amplifier stage (AMPST) further comprises a fourth current path (CP₄) coupled to the second current path (CP₂) for supplying current to the second current path (CP₂) in order to avoid that the output current (Io_(mn)) can be lower than the output reference level (Io_(mn)) when the strength of the input signal (I_(i)) has exceeded the further input reference level (I_(B)).
 8. An optical/magneto-optical disk recording apparatus comprising a light source (LS) for storing data on a disk (DSK), and light-receiving means (PHDS) for the detection of data from the disk (DSK), characterized in that the apparatus comprises an electronic circuit as defined in claim 1, wherein the input signal (I_(i)) of the amplifier stage (AMPST) is responsive to a signal (A; B; C; D) delivered by the light-receiving means (PHDS).
 9. A method whereby an input signal (I_(i)) is converted into an output signal (I_(o)), and whereby the strength of the output signal (I_(o)) increases in response to an increasing strength of the input signal (I_(i)) as long as the strength of the input signal (I_(i)) does not exceed an input reference level (I_(A)), and whereby the strength of the output signal (I_(o)) is kept approximately constant when the strength of the input signal (I_(i)) exceeds the input reference level (I_(A)) but does not exceed a further input reference level (I_(B)), and whereby the strength of the output signal (I_(o)) decreases in response to an increasing strength of the input signal (I_(i)) when the strength of the input signal (I_(i)) exceeds the further input reference level (I_(B)).
 10. A method according to claim 9, characterized in that the strength of the output signal (I_(o)) does not become lower than an output reference level (Io_(mn)) when the strength of the input signal (I_(i)) exceeds the further input reference level (I_(B)). 